Supercavitation occurs when an object moving through water reaches speeds in excess of 100 knots. At this speed it is possible for a bubble of air to form around the object, beginning at the nose of the object. The bubble can extend completely around the entire object and hence the object is no longer moving through the water, rather the object is moving through air. This results in a significantly reduced amount of friction or drag. Hence, supercavitation allows a craft to travel at a high speed on or below the surface of the water with a significant reduction in drag.
When a supersonic airflow passes over a wedge, a shock wave forms at the point of the wedge. This kind of shock wave is called an oblique shock because it forms at some non-orthogonal angle to the surface of wedge (a shock wave perpendicular to the surface is known as a normal shock). As the Mach number increases, the shock angle becomes smaller. Therefore, the distance between the wedge surface and the shock decreases with increasing speed. For a hypersonic body, this distance can become very small over a large portion of the body, and the resulting flow field between the surface and shock is often referred to as a shock layer. The shock layer may merge with the boundary layer at low Reynolds numbers to form a fully viscous shock layer. At high Reynolds numbers, the shock layer can be treated as inviscid (meaning there is no friction). In the limit as Mach number goes to infinity, the shock layer forms an infinitely thin, infinitely dense sheet, or, essentially, a flat plate. The infinite flat plate is the most efficient lifting surface at hypersonic velocities.
Because air is viscous at sub-sonic speeds, any object moving through it collects a group of air particles which it pulls along with it. A particle directly adjacent to the surface of the object will be pulled along at approximately the speed of the object due to viscous adhesion. As an airfoil moves through a free stream of air at a given velocity, this effect causes a very thin layer of air having velocities below that of the free stream velocity, to form upon the airfoil surface. This layer, known as the “boundary layer”, constitutes the interface between the airfoil and its surrounding air mass. Conceptually, the boundary layer may be thought of as the layer of air surrounding an airfoil in which the velocity of the layer of molecules closest to the airfoil is at or near zero with respect to the airfoil, and in which the velocity at successively distant points from the airfoil increases until it approaches that of the free stream, at which point the outer limit of the boundary layer is reached. Generally, boundary layers may be thought of as being one of two types, laminar or turbulent, although there is a region of transition between laminar and turbulent that may, in some cases, be quite large. See FIG. 1 and U.S. Pat. No. 4,802,642 to Mangiarotty which is hereby incorporated by reference. A third condition, in which the boundary layer is “unattached”, must also be recognized. A laminar boundary layer is typified by smooth flow that is free from eddies. Conversely, turbulent flow is characterized by a thicker boundary layer that has a large number of eddies that act to transfer momentum from the faster moving outer portions to the relatively slower portions nearer the airfoil surface. Consequently, a turbulent boundary layer has a greater average velocity near the airfoil surface, and a correspondingly greater amount of surface friction, than does a laminar boundary layer. The increase in surface friction causes increased aerodynamic drag that requires greater power consumption to maintain constant airfoil speed.
Typically, a laminar boundary layer will form at or near the leading edge of a conventional airfoil and extend rearward toward the points of minimum pressure on the upper and lower surfaces. According to Bernoulli's principle, the region between the leading edge and the first minimum pressure point is one of a decreasing pressure gradient. Thereafter, the pressure gradient will increase and the relatively low kinetic energy of the air molecules closest to the airfoil surface may be insufficient to maintain laminar flow against the gradient. In this event it is possible that small perturbations in the boundary layer will develop into eddies that initiate a transition from laminar to turbulent flow. Alternatively, in the presence of higher pressure gradients, the molecules closest to the airfoil surface may actually reverse their direction of motion and begin to move upstream, thereby causing the boundary layer to separate from the airfoil surface. This condition causes significantly more drag, and less lift, than a turbulent boundary layer, and reattachment will not normally occur unless some means is employed to reenergize the boundary layer. The problem, then, is to develop means to control the boundary layer of an airfoil in order to reduce aerodynamic drag and the energy losses associated therewith.
Prevention of the transition from laminar flow to turbulent flow in aerodynamic boundary layers on the surfaces of vehicles is an important method for reducing aerodynamic drag, and hence reducing energy consumption. The invention herein utilizes acoustic energy to increase the incidence of laminar flow. The use of acoustical methods for total or local control of laminar flow is potentially more economical in energy consumption, and also involves simpler and lighter installations than are required for other systems.
In other instances it is desirable to increase drag, for example during vehicle braking. While some aircraft have movable control surfaces that increase drag and lift, movable control surfaces on other vehicles such as automobiles or boats become impractical. Movable control surfaces add considerable weight, cost and complexity to the design of a vehicle, which may nevertheless benefit from increases in drag in certain applications. Aerodynamic drag may be increased by disrupting laminar flows with acoustic energy. Selective radiation of acoustic energy creates a turbulent flow event on a leading aerodynamic edge where an otherwise low drag laminar flow would be present. This disruption of laminar flow with acoustic energy thereby increases vehicle drag. Thus, what is needed is a drag modulation system that uses acoustic energy to increase or decrease an amount of vehicle drag in response to various usages of the vehicle.
A more recent technology involving directional sound has developed as part of an attempt to reproduce sound without use of a moving diaphragm such as is applied in conventional speakers. This sound propagation approach includes technologies embodied in parametric speakers, acoustic heterodyning, beat frequency interference and other forms of modulation of multiple frequencies to generate a new frequency.
In theory, sound is developed by the interaction in air (as a nonlinear medium) of a modulated ultrasonic frequency whose modulation component in value falls within the audio range. The nonlinear characteristics of air under these conditions results in a mixing of the ultrasonically modulated signal at a physical point of contact. The mixing result is the demodulated audio component of the signal. Ideally, resulting compression waves would be projected within the air as a nonlinear medium, and would be heard as pure sound. An interesting property of parametric sound generation is enhanced directionality afforded by the highly directional ultrasonic carrier.
Ultrasonic acoustic energy may be the acoustic energy used to increase and decrease vehicle drag. Ultrasonic energy has the advantage in that the acoustic energy is beyond the hearing range of most individuals, and is thus a quiet mode of drag control. Ultrasonic transducers are tuned to operate efficiently in a relatively narrow frequency range and are typically precluded from being effective at generating frequencies low enough to be heard as audio signals. Since in many applications, it is desirable for a vehicle to emit an audio alert, such as a horn or other warning sound, what is needed is a method and device for both silently modulating the drag of a vehicle and for generating an audio alert when appropriate.
Aerodynamic drag may also be affected by surface properties. A rough surface disrupts laminar now while a smooth surface facilitates laminar flow. Since in various applications it is desirable to either increase or decrease drag, what is needed is a method or device for dispensing a substance or chemical that modifies the surface characteristics of an aerodynamic surface.
Aircraft often experience crosswinds that are tangential winds that have various lift and drag effects. Crosswind results in difficulty in controlling the flight of an aircraft and in providing a comfortable environment for aircraft passengers. Thus, what is needed is a method of modifying the lift and drag of the aircraft in response to the crosswinds.
A slipstream is the turbulent flow of air or water driven backwards by propellers of a craft. A slipstream is also the area of reduced pressure or forward suction produced by immediately behind a fast-moving object as it moves through the air or water. There are a number of ways to affect the slipstream boundary layer or laminar airflow layer either in front or behind the vehicle in order to decrease the turbulence or flow. For example, if three vehicles traveling together in a slipstream with one following another one, all vehicles will travel faster. Thus, it is desirable to improve the slipstream to improve this effect and further to facilitate a virtual vehicle traveling in the slipstream.
Other components of crafts are desirable at certain times and under certain conditions. For example, it may be desirable to lengthen the hull of a sailing ship to increase thrust. It may be desirable to generate an additional control surface or wing of an air craft under certain conditions. Thus, what is needed is a way to create virtual components when needed without creating an actual three dimensional component.
In a multiple mast sailing ship, one mast and sail creates a slipstream. Another mast and sail may be traveling in that slipstream. However, the sails of the multiple masts typically have identical surfaces which are not adapted to take advantage of the slipstream. Thus, what is needed is a sail having surfaces adapted to take advantage of the slipstream.
The thin sails of a sailing ship do not take full advantage of various aerodynamic flows at various operating conditions of the ship because the sails are thin. Thus, what is needed is a sail that may be selectively thickened in response to various sailing conditions.
Keels of sailing ships are typically fixed and unable to modify their hydrodynamic characteristics in response to various sailing conditions. Thus, what is needed is a keel that has modifiable hydrodynamic characteristics.
Internal combustion engines are typically used to generate vehicle thrust. An important component of efficient combustion of an air/fuel mixture in an internal combustion is atomization of the fuel with the air. Since the fuel is mixed with the air at a time very close to the time of combustion, it is important that the atomization process occur quickly. Furthermore, turbulent airflows prior to combustion can disadvantageously cause the fuel to separate from the air. Thus, what is needed is a method or system for facilitating rapid atomization of fuel when mixed with the air and that further deters the fuel from separating from the air under turbulent conditions.
Vehicle tires are an important component of vehicle thrust. The traction of the tire facilitates acceleration, braking and turning. Warm tires have improved traction, but the traction comes at the expense of tread life. There are times when improved traction is preferable to improved tread life, such as competition driving, and there are times when improved tread life is preferable to improved traction. Furthermore, in some instances heating the road surface itself may improved traction, particularly with there are snow or wet condition. Thus, what is needed is a method or system for selectively heating vehicle tires and/or the road surface contacted by the tires.